Process for treating polyethylene structures and articles resulting therefrom



2,715,075 YLENE STRUCTURES THEREFROM 952 WHGl TN NWN@ ...mm2 OPSv. w E@ man., .IS

ETEMv .ALl LECi RI TMF WA Aug. 9, 1955 PROCESN INVENTOR LEON E. WOLINSKI ATTORNEY United States Patent PROCESS FR TREATING PLYETHYLENE STRUCTURES AND ARTHCJES RESULT- ING THEREFRM Leon E. Wolinski, Buffalo, N. Y., assigner to E. l. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application November `29, 1952, Serial No. 323,272

12 Claims. (Cl. 117-47) This invention relates to a process of treating the surface of polyethylene structures and, more particularly,

`to a process of treating the surface of a polyethylene film `high degree of moisture vapor imperrneability, permit the `passage of oxygen, and are heat-sealable.

Because of this combination of properties, polyethylene films are `highly useful for packaging and wrapping a great variety of materials such as chemicals, fresh produce, dried milk, textiles, hardware, etc. Probably the only troublesome disadvantage of polyethylene film for use in the packaging fieldfis the fact that standard aniline and rotogravure printing inks employed for printing various cellulosic films, such as regenerated cellulose and cellulose acetate films, do not adhere satisfactorily to `the surface of the film. Generally, any indicia, such as trademarks, adver- `tising indicia, recipes, etc., imprinted upon a surface of a `polyethylene film with standard `oil or lacquer type inks employed for printing7 cellophane film are. easily smeared ortrubbed off by the normal abrasions suffered by packages during shipping, handling, etc. Hence, in order to obtain satisfactory adhesion between a dried ink and a polyethylene film surface, it is necessary to employ a specially compounded ink or modify the film surface to .promote `improved ink adhesion.

Although printing inks compounded particularly for printing on polyethylene films have been developed, the use of most of these inks requires modification of standard printing processes; and the preferred approach is treatment of ithepolyethylene film surface to `promote adhesion of Astandard oil and lacquer type inks.

An object of the .present invention is to provide a process of treating the surface of a polyethylene structure, e. g., film, `to improve adhesion of standard printing inks, i. e., promote adhesion of standard aniline and rotogravure inks employed in printing on cellophane film. Another object is to provide a process of treating the surface of a polyethylene film to improve adhesion thereof to various `other materials, such as metals, paper, nitrocellulose coatings, and other polymeric coatings, e. g., nylon, polyethylene terephthalate, etc. A further object is to improve the adhesion of polyethylene film to itself and other materials when using commercial adhesives. A still further object is to provide a process of treating the surface of a polyethylene film to improve adhesion thereto of dried ink impressions and not impair the transparency of the film. A still further object is to provide a ,polyethylene film having modified surface characteristics such that dried ink imprints on the surface will not rub off when tested in accordance with the various tests described hereinafter.` Other objects will be apparent from the following description of the invention.

These objects are realized by the present invention 2,715,075 Patented Aug. 9, 1955 'ice which, briefly stated, comprises `subjecting apolyethylene structure, e. g., film, at a temperature `within the range of from about room temperature to a temperature above which substantial degradation of the polyethylene composition occurs, preferably from l50325 C., `to the action of ozone in the presence of a reaction accelerator agent selected from the group consisting of the halogens and the hydrogen halides and, preferably, in the presence of ultra-violet light.

lt should be understood that 325 C. is not an lupper limit insofar as the operability of the present invention is concerned. Extrusion of presently available polyethylene compositions at temperatures substantially higher `than 325 C. is not practical -because the melt is too liuid, and presently known antioxidants do not efciently prevent degradation of the polymerat appreciably higher temperatures. With 4the development `of polyethylene compositions which form a more viscous melt and the discovery of more eihcient antioxidants, extrusion may be carried out more rapidly and efi'iciently at temperatures of 400 C. and above, the maximum being that temperature beyond which substantial degradation of the polyethylene composition occurs.

In the normal process of extrudingmolten polyethylene into film form, a molding powder or flake of polyethylene is fed continuously into a melt extrusion machine, `and the molten film continuously extruded through a slot orifice and through an air `gap vertically downward into a quench bath maintained at `a temperature yfrom 25 C., preferably `from 3060`7C. Usually, the polyethylene is extruded from a melt maintained at a temperature within therange from to 325 C. Tubing is usually extruded from a melt at a temperature Within the range from l50200 C., whereas film is extruded at a temperature within the range from 250-325 C. An alternative process of vforming a polyethylene lm comprises milling molten Vpolymer on. lclosely-spaced calender rolls to form a film `which is conducted vertically downward into a quench bath. In either of these general methods of forming `a polymeric film, the space between the point where the molten film leaves the slot orifice or the `last calender roll and the point where the molten film enters a quench bath will hereinafter be termed the air gap. During passage through the air gap, the film is merely permitted to pass uninhibited through the atmosphere, and this `provides for some superficial cooling. Generally, the length of the air gap ranges Vfrom about 2 toas long as l5 in some cases.

Although it is preferred in most cases and most expedient to `treat the polyethylene film in the air gap, i. e., at l50325 C., the film may be treated at temperatures as low as normal room temperatures so long as the apparatus employed provides for suicient time of treatment. Generally, at lower temperatures, yultra-violet light should be employed as an accelerator along with the halogen and/or halogenacid. The apparatus employed for treating the film at temperatures lower than 150 C. will depend on the degree to which the film is self-supporting, i. e., in order to prevent excessive drawing or change in thickness, the film `may have to be supported during the treatment.

Because of the rapid action of ozone upon the surface of polyethylene film at elevated temperatures, the process of the present invention is most conveniently carried out by subjecting freshly extruded film at a temperature of from about 150 to about 325 C. to the action of ozone as the lm passes through the air gap. For example, `freshly extruded film may be treated in accordance with this invention by suitably enclosing the air gap and providing for the maintenance in the enclosure of a gaseous atmosphere, e. g., air, and containing ozone and the accelerating agent. Provision may be made for .present invention as part of presently employed extrusion or calendering techniques of forming the film, the time of treatment in the air gap should be no greater than about 2 seconds in order to permit operation at commercially acceptable rates. As discussed above, treatment at temperatures lower than 150 C. would require the use of longer treating chambers.

Any accelerator or mixture of accelerators selected from the group consisting of the halogens and the hydrogen halides, e. g., chlorine, bromine, iodine, and fiuorine, and the corresponding hydrogen halides, may be employed with the ozone for purposes of this invention. Preferably, the accelerator may be employed in concentrations equivalent to the mol per cent or volume per cent of ozone present. If greater than equal molar quantities of the halogens are employed, halogenation of the polyethylene will occur. No halogen has been detected in the films treated in accordance with the examples presented hereinafter.

For effective action within the relatively short exposure times permissible in the treatment of film being extruded at commercially acceptable rates, the concentration of ozone in the treating atmosphere must be at least 0.01% by volume, of the total volume of gases present in the treating gases and, preferably, about 0.1%. The use of ozone concentrations substantially greater than 5%, by volume of the total gases surrounding the film, is not particularly practical because of the restricted capacity of present-day ozone generating equipment. This applies to continuous treatment of film wherein the ozone-containing gas, e. g., air, is passed continuously through the treating chamber and additional ozone is injected into the out gases which are then recirculated. Ozone concentrations as high as do not tend to burn the film, i. e., impair transparency or semitransparency of the film, so long as the time of treatment or exposure is not excessive.

Preferably, ultra-violet light having a wave length no greater than 3900 U is employed to accelerate the action of ozone, particularly where the process of this invention is incorporated in the continuous production of film.

The following examples illustrate the preferred practice of this invention, reference being had to the accompanying drawing wherein is shown diagrammatically an arrangement of apparatus used in carrying out the process of this invention.

Referring to the drawing, molten polyethylene at a temperature of 265 C., was extruded in the form of a film F from extrusion hopper 1 into the air gap surrounded by an enclosure constituting a treating chamber 2, the walls of which are formed in part, at least, of transparent material (quartz glass) to provide for the transmission therethrough of ultra-violet light emitted'from mercury arc lamps 3 placed two inches from the film. The upper end of the chamber was closed by the extrusion hopper, and the bottom of the chamber was sealed from the atmosphere by projecting the sides thereof below the surface of the cooling water (60 C.) in the quench bath 4. The length of the air gap was 10 inches, and the path of travel of the film in the quench bath was also 10 inches. Air at atmospheric pressure and containing ozone and accelerator was passed into the chamber at 5 and out at 6. At no time did the temperature of film in chamber 2 drop below 200 C. The treatment times set forth in la the following table are the actual times that any given increment of the film remained in the treating chamber. The concentration of the accelerators was equivalent to that of ozone. ln every example, the printability of the treated film was excellent.

Table I Accelerator (U V ultraviolet light) Ozone Qoneentration, percent Reaction Time, sec.

In evaluating the printability, i. e., the strength of the adhesive bond between the dried ink and the treated polyethylene film surface, a number of tests were employed (5 in all); and on the basis of the results of all of the tests, the films were rated either acceptable or not acceptable, and, if acceptable, either excellent or good. Four different inks Were employed to print the treated surfaces of polyethylene film, and each printed sample was evaluated in accordance with each of the five tests which will be described hereinafter. The inks employed were as follows:

No. 1.-Aniline cellophane ink (Bensing Bros. and Deeney, No. W-400).

No. 2.-Aniline polyethylene ink (Interchemical Corporation, No. PA-Red).

No. 3.-Rotogravue cellophane ink (Bensing Bros. and Deeney, No. G-1037).

No. 4.-Rotogravure polyethylene ink (Interchemical Corporation, IN-Tag-Red, GPA Red).

In preparing the printed samples of polyethylene film, the ink was applied with a commercial ink spreader which comprised a steel rod having fine wire Wrapped around the rod. The spreader produced a multiplicity of fine lines. The ink was then dried for three minutes at C. and thereafter permitted to cool to room temperature. Each sample was then tested in accordance with each of the following tests, and the amount of ink rubbed off and/ or removed was noted:

1. Rub test.-The inked polyethylene surface was rubbed ten times against a hard white paper.

2. Scratch test-The back of a fingernail was rubbed across the inked surface.

3. Flex test- The film was held between thumb and forefinger (2 apart) and flexed vigorously.

4. Pressure-sensitive tape test2-A pressure-sensitive tape was pressed against the printed surface, and then pulled off.

5. Twist test.-The printed film was folded once and then again in a direction perpendicular to the first fold. The folded ends ware then twisted once around, and thereafter the film surface was examined for smearing and/ or cracking of the dried ink.

Although it is convenient and preferable to treat the polyethylene film with ozone and an accelerator in the air gap between the extrusion orifice and the quench bath as described above, or between the last calender roll and the quench bath, the film being at a temperature close to the actual melt extrusion temperature, the process of the present invention may be applied to the film at a point after the film has been quenched. In carrying vout the process of the present invention at this point the film was conducted through a reaction chamber wherein the film was maintained at a temperature between room temperature and about 325 C., and was subjected to a confined atmosphere containing ozone and an accelerator of the class hereinabove defined. The conditions employed and the results of the treatment are set forth in the following table. In every example, the printability of the treated film was excellent.

Table Il O zone Conceutration percent Example Reaction Time NNNNKINNINN While the` present .process is employed primarily for treating the surface of a polyethylene film in order to produce a film which may be successfully printed with standard oil or lacquer type inks, e. g., aniline or rotogravure inks employed for printing on cellophane film, the present invention may also be employed to modify the surface of a polyethylene film which is to` be printed with inks which are especially modified for printing upon a polyethylene `film surface. The net result is an even further improvement in the adhesive bonds between the dried ink and the polyethylene film surface. The present invention further provides for the preparation of a polyethylene lm which is more readily adherent to metals, papers, and various coatings, such as those of nitrocellulose; polyamides, e. g., polyhexamethylene adipamide, polyhexamethylene sebacamide, N-methoxymethyl polyhexamethylene adipamide and other polyamides defined in U. S. P. 2,430,860, and interpolyamides defined in U. S. P. 2,285,009; polyethylene terephthalate; polyvinyl acetals such as polyvinyl butyral; ethyl cellulose; vinyl acetate-vinyl chloride copolymers; vinylidene chloride copolymers; chlorinated rubbers; etc. Furthermore, polyethylene film treated by the present process is more readily adhered to itself and other base materials by using commercial adhesives, c. g., standard adhesives employed for sealing cellophane.

The process of this invention may also be employed for treating the surface of various films fabricated from copolymers of ethylene with various other polymerizable materials, e. g., isobutylene, vinyl acetate, styrene, vinyl chloride.

The outstanding advantage of the present process is that it provides a readily applicable and rapid method of improving the adhesion of a dried printing ink to the surface of a polyethylene film. The process may be readily combined with a necessary step of extruding or calendering molten polyethylene into film or tube form, and the additional apparatus required is inexpensive and easy to install.

Another outstanding advantage of the present inventioin is that it provides for the preparation of an im proved polyethylene film which forms heat seals of higher bond strength than seals made with films treated by any other known process of improving the adhesion of printing inks to` polyethylene film. This is especially true of film which has been treated with a sizing composition, such as an aqueous solution of an alkyl aryl polyglycol ether of the type defined in U. S. P. 2,519,013.

As many widely different embodiments may be made without departing from the spirit and scope of this invention, it is to be understood that said invention is in no wise restricted except as set forth in the appended claims.

I claim:

l. A process for treating structures of polyethylene which comprises subjecting said structures to the action of a gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens tit and the hydrogen halides, at a temperature Within the range of from room vtemperature to the temperature beyond which substantial degradation `of the polyethylene occurs, and for a period of time sufficient to render said structures adherent to printing ink.

2. A process for treating structures of polyethylene which comprises subjecting said structures to the action of a gaseous atmosphere containing at. least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and the hydrogen halides, at a temperature within the range of from about to about 325 C., and for a period of time sufficient to `render saidstructures adherent .to `printing ink.

3. A process for treating structures of polyethylene which comprises subjecting `said structures to the action of a `gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, .not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and the hydrogen halides, in the presence of ultraviolet light having a -wave length no greater than 3900 U, at a temperature within the range of from room `temperature tothe temperature beyond which substan- `to printing ink.

4. A process for treating structures of polyethylene which comprises subjecting said structures to the action of a gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and hydrogen halides, in the presence of ultra-violet light, at a temperature within the range of from about 150 to about 325 C., and for a period of time sufficient to render said structures adherent to printing ink.

5. A process for treating polyethylene film which comprises subjecting the surface of said film to the action of gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and the hydrogen halides, at a temperature within the range of from room temperature to the temperature beyond which substantial degradation of the polyethylene occurs, and for a period of time sufficient to render said surface adherent to printing ink.

6. A polyethylene film resulting from the process defined in claim 5.

7. A process for treating polyethylene film which comprises subjecting the surface of said film to the action of a gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and the hydrogen halides, at a temperature within the range of from about 150 to about 325 C., and for a eriod of time sufiicient to render said surface adherent to printing ink.

8. A process for treating polyethylene lm which comprises subjecting the surface of said film to the action of a gaseous atmosphere containing at least 0.01% by volurne of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and the hydrogen halides, in the presence of ultra-Violet light having a wave length no greater than 3900 U, at a temperature within the range of from about 150 to about 325 C., and for a period of time sufiicient to render surface adherent to printing ink.

9. A process for treating polyethylene film which comprises passing continuous film of polyethylene continuously through a zone wherein said film is maintained at a temperature within the range of from room temperature to the temperature beyond which substantial degradation ofthe polyethylene occurs, and subjecting the surface of the film in said zone to the action of a gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and the hydrogen halides, for a period of time sufficient to render said surface adherent to printing ink.

10. A process for treating polyethylene lm which comprises passing continuous ilm of polyethylene continuously through a zone wherein said film is maintained at a temperature within the range of from about 150 to about 325 C., and subjecting the surface of the iilmy in said zone to the action of a gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent seelcted from the group consisting of the halogens and the hydrogen halides, for a period of time sufficient to render said surface adherent to printing ink.

11. A process for treating polyethylene film which comprises passing continuous film of polyethylene continuously through a zone wherein said film is maintained at a temperature Within the range of from about 150 to about 325 C., and subjecting the surface of the lm in said zone to the action of a gaseous atmosphere containing at least 0.01 by volume of ozone and an amount,

y of a gaseous atmosphere containing at least 0.01% by volume of ozone and an amount, not in excess of the volume per cent of ozone present, of a gaseous accelerator agent selected from the group consisting of the halogens and the hydrogen halides, at a temperature within the range of from room temperature to the temperature beyond which substantial degradation of the polyethylene occurs, and for a periodof time suicient to render said surface adherent to printing ink, and thereafter imprinting such surface with a printing ink.

References Cited in the le of this patent UNITED STATES PATENTS 2,400,720 Staudinger May 21, 1946 2,499,421 Samler Mar. 7, 1950 2,502,841 Henderson Apr. 4, 1950 2,612,480 May Sept. 30, 1952 2,622,056 De Couderes Dec. 16, 1952 2,639,998 Pavlic May 26, 1953 

12. A PROCESS FOR TREATING POLYETHYLENE FILM WHICH COMPRISES SUBJECTING THE SURFACE OF SAID FILM TO THE ACTION OF A GASEOUS ATMOSPHERE CONTAINING AT LEAST 0.01% BY VOLUME OF OZONE AND AN AMOUNT, NOT IN EXCESS OF THE VOLUME PER CENT OF OZONE PRESENT, OF A GASEOUS ACCELERATOR AGENT SELECTED FROM THE GROUP CONSISTING OF THE HALOGENS AND THE HYDROGEN HALIDES, AT A TEMPERATURE WITHIN THE RANGE OF FROM ROOM TEMPERATURE TO THE TEMPERATURE BEYOND WHICH SUBSTANTIAL DEGRADATION OF THE POLYETHYLENE OCCURS, AND FOR A PERIOD OF TIME SUFFICIENT TO RENDER SAID SURFACE ADHERENT TO PRINTING INK, AND THEREAFTER IMPRINTING SUCH SURFACE WITH A PRINTING INK. 